Biochemical and enzymatic characterization of a partially purified casein kinase-1 like activity from Trypanosoma cruzi

Enzyme Activity Assays for Protein Kinases: Strategies to Identify Active Substrates

Protein kinases are an important class of enzymes and drug targets. New opportunities to discover medicines for neglected diseases can be leveraged by the extensive kinase tools and knowledge created in targeting human kinases. A valuable tool for kinase drug discovery is an enzyme assay that measures catalytic function. The functional assay can be used to identify inhibitors, estimate affinity, characterize molecular mechanisms of action (MMOAs) and evaluate selectivity. However, establishing an enzyme assay for a new kinases requires identification of a suitable substrate. Identification of a new kinase's endogenous physiologic substrate and function can be extremely costly and time consuming. Fortunately, most kinases are promiscuous and will catalyze the phosphotransfer from ATP to alternative substrates with differing degrees of catalytic efficiency. In this manuscript we review strategies and successes in the identification of alternative substrates for kinases from organisms responsible for many of the neglected tropical diseases (NTDs) towards the goal of informing strategies to identify substrates for new kinases. Approaches for establishing a functional kinase assay include measuring auto-activation and use of generic substrates and peptides. The most commonly used generic substrates are casein, myelin basic protein, and histone. Sequence homology modeling can provide insights into the potential substrates and the requirement for activation. Empirical approaches that can identify substrates include screening of lysates (which may also help identify native substrates) and use of peptide arrays. All of these approaches have been used with a varying degree of success to identify alternative substrates.

CK2 Secreted by Leishmania braziliensis Mediates Macrophage Association Invasion: A Comparative Study between Virulent and Avirulent Promastigotes

CK2 is a protein kinase distributed in different compartments of Leishmania braziliensis: an externally oriented ecto-CK2, an intracellular CK2, and a secreted CK2. This latter form is constitutively secreted from the parasite (CsCK2), but such secretion may be highly enhanced by the association of specific molecules, including enzyme substrates, which lead to a higher enzymatic activity, called inductively secreted CK2 (IsCK2). Here, we examined the influence of secreted CK2 (sCK2) activity on the infectivity of a virulent L. braziliensis strain. The virulent strain presented 121-fold higher total CK2 activity than those found in an avirulent strain. The use of specific CK2 inhibitors (TBB, DRB, or heparin) inhibited virulent parasite growth, whereas no effect was observed in the avirulent parasites. When these inhibitors were added to the interaction assays between the virulent L. braziliensis strain and macrophages, association index was drastically inhibited. Polyamines enhanced sCK2 activity and increased the association index between parasites and macrophages. Finally, sCK2 and the supernatant of the virulent strain increased the association index between the avirulent strain and macrophages, which was inhibited by TBB. Thus, the kinase enzyme CK2 seems to be important to invasion mechanisms of L. braziliensis.

Pharmacological assessment defines Leishmania donovani casein kinase 1 as a drug target and reveals important functions in parasite viability and intracellular infection

Protein kinase inhibitors have emerged as new drugs in various therapeutic areas, including leishmaniasis, an important parasitic disease. Members of the Leishmania casein kinase 1 (CK1) family represent promising therapeutic targets. Leishmania casein kinase 1 isoform 2 (CK1.2) has been identified as an exokinase capable of phosphorylating host proteins, thus exerting a potential immune-suppressive action on infected host cells. Moreover, its inhibition reduces promastigote growth. Despite these important properties, its requirement for intracellular infection and its chemical validation as a therapeutic target in the disease-relevant amastigote stage remain to be established. In this study, we used a multidisciplinary approach combining bioinformatics, biochemical, and pharmacological analyses with a macrophage infection assay to characterize and define Leishmania CK1.2 as a valid drug target. We show that recombinant and transgenic Leishmania CK1.2 (i) can phosphorylate CK1-specific substrates, (ii) is sensitive to temperature, and (iii) is susceptible to CK1-specific inhibitors. CK1.2 is constitutively expressed at both the promastigote insect stage and the vertebrate amastigote stage. We further demonstrated that reduction of CK1 activity by specific inhibitors, such as D4476, blocks promastigote growth, strongly compromises axenic amastigote viability, and decreases the number of intracellular Leishmania donovani and L. amazonensis amastigotes in infected macrophages. These results underline the potential role of CK1 kinases in intracellular survival. The identification of differences in structure and inhibition profiles compared to those of mammalian CK1 kinases opens new opportunities for Leishmania CK1.2 antileishmanial drug development. Our report provides the first chemical validation of Leishmania CK1 protein kinases, required for amastigote intracellular survival, as therapeutic targets.

Mammalian casein kinase 1alpha and its leishmanial ortholog regulate stability of IFNAR1 and type I interferon signaling

Phosphorylation of the degron of the IFNAR1 chain of the type I interferon (IFN) receptor triggers ubiquitination and degradation of this receptor and, therefore, plays a crucial role in negative regulation of IFN-alpha/beta signaling. Besides the IFN-stimulated and Jak activity-dependent pathways, a basal ligand-independent phosphorylation of IFNAR1 has been described and implicated in downregulating IFNAR1 in response to virus-induced endoplasmic reticulum (ER) stress. Here we report purification and characterization of casein kinase 1alpha (CK1alpha) as a bona fide major IFNAR1 kinase that confers basal turnover of IFNAR1 and cooperates with ER stress stimuli to mediate phosphorylation-dependent degradation of IFNAR1. Activity of CK1alpha was required for phosphorylation and downregulation of IFNAR1 in response to ER stress and viral infection. While many forms of CK1 were capable of phosphorylating IFNAR1 in vitro, human CK1alpha and L-CK1 produced by the protozoan Leishmania major were also capable of increasing IFNAR1 degron phosphorylation in cells. Expression of leishmania CK1 in mammalian cells stimulated the phosphorylation-dependent downregulation of IFNAR1 and attenuated its signaling. Infection of mammalian cells with L. major modestly decreased IFNAR1 levels and attenuated cellular responses to IFN-alpha in vitro. We propose a role for mammalian and parasite CK1 enzymes in regulating IFNAR1 stability and type I IFN signaling.

Inhibitors of casein kinase 1 block the growth of Leishmania major promastigotes in vitro

Casein kinase 1 (CK1) is a family of multifunctional Ser/Thr protein kinases that are ubiquitous in eukaryotic cells. Recent studies have demonstrated the existence of, and role for, CK1 in protozoan parasites such as Leishmania, Plasmodium and Trypanosoma. The value of protein kinases as potential drug targets in protozoa is evidenced by the successful exploitation of cyclic guanosine monophosphate-dependent protein kinase (PKG) with selective tri-substituted pyrrole and imidazopyridine inhibitors. These compounds exhibit in vivo efficacy against Eimeria tenella in chickens and Toxoplasma gondii in mice. We now report that both of these protein kinase inhibitor classes inhibit the growth of Leishmania major promastigotes and Trypanosoma brucei bloodstream forms in vitro. Genome informatics predicts that neither of these trypanosomatids codes for a PKG orthologue. Biochemical studies have led to the unexpected discovery that an isoform of CK1 represents the primary target of the pyrrole and imidazopyridine kinase inhibitors in these organisms. CK1 from extracts of L. major promastigotes co-fractionated with [(3)H]imidazopyridine binding activity. Further purification of CK1 activity from L. major and characterization via liquid chromatography coupled tandem mass spectrometry identified CK1 isoform 2 as the specific parasite protein inhibited by imidazopyridines. L. major CK1 isoform 2 expressed as a recombinant protein in Escherichia coli displayed biochemical and inhibition characteristics similar to those of the purified native enzyme. The results described here warrant further evaluation of the activity of these kinase inhibitors against mammalian stage Leishmania parasites in vitro and in animal models of infection, as well as studies to genetically validate CK1 as a therapeutic target in trypanosomatid parasites.

Tight binding between a pool of the heterodimeric α/β tubulin and a protein kinase CK2 inTrypanosoma cruziepimastigotes

Tubulin is the predominant phosphoprotein inTrypanosoma cruziepimastigotes and is phosphorylated by a protein kinase CK2. Interestingly, the presence or absence of divalent cations affected the solubilization of a pool of the parasite tubulin and the CK2 responsible for its phosphorylation. This fraction of tubulin and its kinase co-eluted using phosphocellulose, DEAE-Sepharose and Sephacryl S-300 chromatographies. Anti-α tubulin antibodies co-immunoprecipitated both tubulin and the CK2 responsible for its phosphorylation, and anti-CK2 α-subunit antibodies immunoprecipitated radioactively labelled α and β tubulin from phosphorylated epimastigote homogenates. Additionally, native polyacrylamide gel electrophoresis of the purified and radioactively labelled fraction containing tubulin and its kinase demonstrated the phosphorylation of a unique band that reacted with both anti-CK2 α-subunit and anti-tubulin antibodies. Together, these results establish a strong interaction between a pool of the heterodimeric α/β tubulin and a CK2 in this parasite. Hydrodynamic measurements indicated that theT. cruzitubulin-CK2 complex is globular with an estimated size of 145·4–147·5 kDa.

Characterization of two T. gondii CK1 isoforms

Previous affinity chromatography experiments have described the unexpected binding of an isoform of casein kinase I (CK1) from Leishmania mexicana, Trypanosoma cruzi, Plasmodium falciparum and Toxoplasma gondii to an immobilized cyclin-dependent kinase (CDK) inhibitor (purvalanol B). In order to further evaluate CK1 as a potential anti-parasitic target, two T. gondii CK1 genes were cloned by PCR using primers derived from a putative CK1 gene fragment identified from a T. gondii EST database. The genes are predicted to encode a smaller polypeptide of 38 kDa (TgCK1alpha) and larger 49 kDa isoform bearing a C-terminal extension (TgCK1beta). Enzymatically active recombinant FLAG-epitope tagged TgCK1alpha and TgCK1beta enzymes were immuno-precipitated from transiently transfected T. gondii parasites. While TgCK1alpha expression was found to be cytosolic, TgCK1beta was expressed predominantly at the plasma membrane. Deletion mapping showed that the C-terminal domain of TgCK1beta confers this membrane-association. Recombinant TgCK1alpha and TgCK1beta isoforms were also expressed in E. coli and biochemically characterized. A 38kDa native CK1 activity was partially purified from T. gondii tachyzoites by ion-exchange and hydrophobic interaction chromatography with biochemical and serological properties closely resembling those of recombinant TgCK1alpha. In contrast, we were not able to identify a native CK1 activity corresponding to the larger TgCK1beta 49 kDa isoform in tachyzoite lysates. Purvalanol B and the related compound aminopurvalanol A selectively inhibit TgCK1alpha, confirming the existence of potentially exploitable structural differences between host and parasite CK1 enzymes. Since the more cell-permeable aminopurvalanol also inhibits parasite growth, these results provide further impetus to investigate inhibitors of CK1 as anti-parasitic agents.

Identification of a protein kinase which phosphorylates a subunit of the 26S proteasome and changes in its activity during meiotic cell cycle in goldfish oocytes

The proteasome is involved in the progression of the meiotic cell cycle in fish oocytes. We reported that the alpha4 subunit of the 26S proteasome, which is a component of the outer rings of the 20S proteasome, is phosphorylated in immature oocytes and dephosphorylated in mature oocytes. To investigate the role of the phosphorylation, we purified the protein kinase from immature oocytes using a recombinant alpha4 subunit as substrate. A protein band which well corresponded to the kinase activity was identified as casein kinase Ialpha (CKIalpha). Two-dimensional (2D) PAGE analysis showed that part of the alpha4 subunit was phosphorylated by CKIalpha in vitro. This spot was detected in purified immature 26S proteasome but not in mature 26S proteasome, demonstrate that the alpha4 subunit is phosphorylated by CKIalpha meiotic cell cycle dependently.

Identification of casein kinase 1, casein kinase 2, and cAMP-dependent protein kinase-like activities in Trypanosoma evansi

Trypanosoma evansi contains protein kinases capable of phosphorylating endogenous substrates with apparent molecular masses in the range between 20 and 205 kDa. The major phosphopolypeptide band, pp55, was predominantly localized in the particulate fraction. Anti-alpha and anti-beta tubulin monoclonal antibodies recognized pp55 by Western blot analyses, suggesting that this band corresponds to phosphorylated tubulin. Inhibition experiments in the presence of emodin, heparin, and 2,3-bisphosphoglycerate indicated that the parasite tubulin kinase was a casein kinase 2 (CK2)-like activity. GTP, which can be utilized instead of ATP by CK2, stimulated rather than inactivated the phosphorylation of tubulin in the parasite homogenate and particulate fraction. However, GTP inhibited the cytosolic CK2 responsible for phosphorylating soluble tubulin and other soluble substrates. Casein and two selective peptide substrates, P1 (RRKDLHDDEEDEAMSITA) for casein kinase (CK1) and P2 (RRRADDSDDDDD) for CK2, were recognized as substrates in T. evansi. While the enzymes present in the soluble fraction predominantly phosphorylated P1, P2 was preferentially labeled in the particulate fractions. These results demonstrated the existence of CK1-like and CK2-like activities primarily located in the parasite cytosolic and membranous fractions, respectively. Histone II-A and kemptide (LRRASVA) also behaved as suitable substrates, implying the existence of other Ser/Thr kinases in T. evansi. Cyclic AMP only increased the phosphorylation of histone II-A and kemptide in the cytosol, demonstrating the existence of soluble cAMP-dependent protein kinase-like activities in T. evansi. However, no endogenous substrates for this enzyme were identified in this fraction. Further evidences were obtained by using PKI (6-22), a reported inhibitor of the catalytic subunit of mammalian cAMP-dependent protein kinases, which specifically hindered the cAMP-dependent phosphorylation of histone II-A and kemptide in the parasite soluble fraction. Since the sum of the values obtained in the parasite cytosolic and particulate fractions were always higher than the values observed in the total T. evansi lysate, the kinase activities examined here appeared to be inhibited in the original extract.

Protein kinase CK1 from Trypanosoma cruzi

A protein kinase activity, which uses casein as a substrate, has been purified to homogeneity from the epimastigote stage of Trypanosoma cruzi, by sequential chromatography on Q sepharose, heparin sepharose, phenyl sepharose, and alpha-casein agarose. An apparent molecular weight of 36,000 was estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Gel filtration chromatography and sedimentation analyses demonstrated that the purified native enzyme is a monomer with a sedimentation coefficient of 2.9 S. The hydrodynamic parameters indicated that the shape of the protein is globular with a frictional ratio f/f(o) = 1.36 and a Stokes radius of 27.7 A. When two selective peptide substrates for protein kinases CK1 and CK2 were used (RRKDLHDDEEDEAM. SITA and RRRADDSDDDDD, respectively), the purified kinase was shown to predominantly phosphorylate the CK1-specific peptide. Additionally, the enzyme was inhibited by N-(2-amino-ethyl)-5-chloroisoquinoline-8-sulfonamide, a specific inactivator of CK1s from mammals. Based on these results, we concluded that the purified kinase corresponds to a parasite CK1.

Trypanosoma cruzi: in vitro phosphorylation of tubulin by a protein kinase CK2-like enzyme

One predominant 55-kDa polypeptide was phosphorylated in vitro in Trypanosoma cruzi homogenates prepared from three differentiation stages: epimastigotes, trypomastigotes, and spheromastigotes. Anti-alpha and anti-beta tubulin monoclonal antibodies immunoprecipitated the phosphorylated 55-kDa polypeptide from epimastigote extracts. Phosphoserine was the only residue phosphorylated in vitro in the 55-kDa polypeptide and in immunoprecipitated alpha tubulin. The phosphorylation of both the 55-kDa polypeptide and exogenously added casein was inhibited with GTP, heparin, and 2,3-bisphosphoglycerate in a dose-dependent manner, indicating the involvement of a CK2-like protein kinase. Moreover, when tubulin was isolated from an epimastigote homogenate by ultracentrifugation, followed by DEAE-Sephacel chromatography, a protein kinase that phosphorylated tubulin and casein co-purified with this cytoskeletal component. This result suggests an association between tubulin and its corresponding protein kinase in T. cruzi.